The soft palate is an extension of the hard palate that divides the oral cavity and the nasal cavity. The soft palate forms a seal with the base of the epiglottis which is a triangular shaped piece of tissue that flips up to protect the airway during swallowing so that food does not enter the trachea and progress down into the lungs. The diagram below shows the soft palate, epiglottis and their relationship with the airway (Figure 1a). During exercise the soft palate moves down and the airway opens up to maximise the amount of room for air to pass into the lungs. Importantly the epiglottis sits firmly on top of the soft palate maintaining that seal between the oral and nasal cavities and allowing the horse to breathe entirely through its nose. Figure 1b shows a dorsally displaced pallet with evidence of chronic ulceration along the caudal border.
Permanent DDSP can be induced by blocking the pharyngeal branch of the vagus nerve which runs within the horses’ guttural pouch (middle ear). Inflammation of this region has been postulated as a possible cause of the intermittent form of DDSP and therefore a possible neuromuscular cause is a likely contributing factor to the condition. Excessive caudal (backwards) retraction of the larynx by contraction of the two muscle groups that attach to the laryngeal region (sternohyoideus and sternothyroideus), excessive movement of the tongue, opening of the mouth or swallowing during exercising and palatal cysts can all cause DDSP. Flaccidity of the epiglottis or an overly long soft palate has been implicated although scientific evidence for this is lacking.
In some horses during exercise the soft palate can move up (dorsal) to lie on top of the epiglottis, thereby displacing and preventing the seal between the oral and nasal cavity from remaining intact. As you will see in Figure 1b, when this happens the soft palate ends up obstructing the opening to the trachea (larynx) and thereby reduces the amount of air that a horse can take in. The soft palate typically will billow up and down and, rather like a flapping sail in the wind, resulting in a gurgling like noise that is heard most commonly during expiration. This can also sound like the horse is choking during exercise. Typically when this happens the horse will lose speed and in some cases will practically stop as they struggle to get enough air. When the horse swallows, the soft palate and epiglottis relationship usually returns to normal and the horse can then proceed without difficulty.
Intermittent displacement of the soft palate is most common, although in some rare cases a more permanent form of displacement can occur. In these cases horses often will cough.
The diagnosis of permanent dorsal displacement of the soft palate is easily made by standing endoscopic examination of the upper airway. The appearance as seen above is typical with the epiglottis lying on top of the soft palate. The more common intermittent displacement of the soft palate is often suspected based on the history of the horse making a characteristic gurgling sound during exercise combined with a history of the horse stopping during a race. Confirmation of this condition is best made with the use of overground endoscopy in which an endoscope is placed up the nostril to view the respiratory tract as the horse is exercised on their own or alongside another horse. Even then this condition can be difficult to replicate as the pressures of the racing environment are often hard to recreate on a standard exercising gallop. If the soft palate displaces during exercise and is not immediately replaced then a diagnosis of dorsal displacement of the soft palate can be made and steps aimed towards treatment can begin.
It should be noted that at rest, many horses will displace their soft palate. This is normal unless they are having difficulty in replacing it. In addition, a sedated horse will displace their soft palate and it can remain displaced for some time - this is not confirmation of a pathological problem.
Following surgery horses are typically maintained on antibiotics and anti-inflammatories for a short period of time combined with a short period of rest varying between 2 and 4 weeks depending on the procedure. Often the training aids discussed above are included in post-operative management in training and racing.
This condition can be frustrating to correct and may recur even after an initial period of success. This underlines the difficulty in knowing an exact cause in the majority of horses that are affected. After a thorough examination of the upper airway under various conditions, your veterinarian will be able to recommend the treatment option most likely to have the best impact on your horse.